In safety-oriented industries, especially in the chemical industry or in power stations, safety valves have to be tested regularly with respect their functionality. Safety valves are used only rarely because they are intended only for the case of emergency.
Pneumatic drives are known which serve as an actuator of a safety valve. As a rule, safety valves are controlled in a safety circuit by means of a magnetic valve such that the same vents the pneumatic drive in the case of emergency by a voltage drop of the supply voltage such that the safety valve is moved into a safety position. The movement into the safety position is effected, in a manner known per se, by means of an energy memory which is realised by springs arranged in the drive means.
For correct function of the safety valve, the actual actuator as well as the magnetic valve have to operate reliably. For monitoring the functionality of an actuator in an actuation system, the so called partial-stroke-test method is known.
Such a partial-stroke-test method is described, for example, in DE 10 2005 004 477 A1. In this method, the actuator is moved briefly by its drive element over a portion of its actuating path to test its functionality. Overshooting is prevented thereby that the drive element can be moved, for example by means of a stop, only within permitted limits. Known partial-stroke-test methods use a position controller in order to determine the correct functionality of the actuator by means of partial movements.
A further device for monitoring a safety valve is known from GB 2 372 087 A. In this method, a control valve is moved up to a predetermined point by means of a test method. The test is started by a laptop or computer. For this purpose, a magnetic valve is shut down like in a case of emergency, but switched on again after a predetermined time. Pressure sensors or flow sensors determine the functionality of the safety valve based on changes in the process medium, and a failure notification is displayed upon occurrence of a failure by means of lamps or a display screen.
By means of this test, it can be tested whether a valve is unduly blocked, whether a spring of the actuator or the valve, respectively, is broken, whether corrosion is present at the valve ball or whether an undesired crystal formation is present.
All these tests pertain to the functionality of the actuator and they are not adapted to check a failure function of the magnetic valve associated with the actuator.
From WO 2007/087030 A1, a device is known which checks a failure of the magnetic valve. The device comprises, for this purpose, a position controller which comprises two outputs—double acting controller—and two pressure sensors. The magnetic valve is connected to the position controller of the actuator through a first pneumatic conduit and a first pressure sensor being provided at the position controller. The magnetic valve is connected to the pneumatic drive by means of a second pneumatic conduit in which a second pressure sensor is arranged at the position controller. The device comprises a circuit for controlling a single-acting pneumatic drive. The circuit provides for connecting the second output of the position controller which is normally not used in this case, with the pneumatic conduit between the magnetic valve and the driving volume. For testing the magnetic valve, the valve is momentarily shut down such that a pressure change or a differential pressure change, respectively, of the pressure sensors in the position controller can be evaluated. Depending on the magnitude of the measured pressure change of the pressure in the pneumatic drive or in the pneumatic conduit between the magnetic valve and the pneumatic drive, respectively, the position controller can decide on the functionality of the magnetic valve.
The requirement of a double acting position controller for a single acting drive is disadvantageous in this arrangement. A single acting position controller for a single acting drive, however, would be advantageous. Furthermore, a reliable pneumatic connection of the second output of the position controller is necessary and, thereby, increases the complexity when installing the pneumatic connection conduits. Furthermore, each additional pneumatic conduit forms a possible source of failure.
Furthermore, if no pressure sensor is provided between the magnetic valve and the pneumatic drive, there is the problem that a momentary pressure change can not be determined during switching the magnetic valve on and off. In particular, a single acting position controller even if it comprises a pressure sensor for measuring the pressure of the pneumatic output, can not determine a pressure change between the magnetic valve and the pneumatic drive during switching the magnetic valve on and off, since the switching of the magnetic valve interrupts the pneumatic connection between the position controller and the pneumatic drive.